This invention relates to a rotary fluid pump or compressor having side plates which are relatively free from wear to provide good outwardly radial sliding movement of the vanes and good sealability and, more particularly, to a rotary fluid pump or compressor having its side plates formed of a material having a wear resistance higher than that of the rotor.
A nagging and persistant problem in the design of rotary pumps and compressors has been to adequately seal the axial ends of the working chamber at the sliding interfaces between the rotor and the stator housing. Any leakage at such seals tends to compromise the pump efficiency or compression ratio, and the problem is particularly onerous owing to the axial expansion pressures developed in the working chamber during operation.
U.S. Pat. No. 2,702,509 attempts to provide a rotary pump having a pair of resilient sealing membranes disposed at the opposite ends of the rotor to prevent fluid leakage from the end faces thereof. Since the membranes do not follow the axial movement or inclination of the end faces of the rotor, however, satisfactory sealing is not always obtained.
To overcome this drawback, according to U.S. Pat. Nos. 2,558,837 and 2,833,465, loose pads are provided at the end faces of the rotor and are urged there against by biasing springs. The pads tightly contact the rotor and may rotate together therewith, however, and the rotor shaft extending through receiving holes in the pads tends to damage and wear them.
In U.S. Pat. No. 3,695,791 a pair of bimetals are used as sealing plates to eliminate any gaps between the plates and the rotor end faces. It takes some time for the bimetals to properly thermally deform, however, whereby effective sealing is not always obtained, particularly during the initial startup period.
In order to overcome the above-mentioned drawbacks and disadvantages, the present applicant designed an improved rotary pump or compressor which is the subject of U.S. application Ser. No. 637,459, filed Dec. 3, 1975, and assigned to the assignee of this application. As shown in FIG. 1, the pump or compressor according to the prior application includes a stator housing 1 and a pair of end heads 2, 3 having recesses 2a, 3a therein assembled to form a pump cavity in which a rotor 6" is disposed in a cantilevered manner on the end of a drive shaft 5 eccentrically journalled in the end head 3. A pair of flexible side plates 7', 8' are sealingly disposed between the side walls of the stator housing and the respective end heads 2, 3 to thereby divide the pump cavity into a pair of end chambers 9, 10 defined by the plates 7', 8' and recesses 2a, 3a and a rotor or working chamber 4. Pressurized air may be supplied to the end chambers to establish a positive pressure differential with respect to the working chamber, whereby the side plates are urged into contact with the end faces of the rotor to maintain a satisfactory working seal. When the structure is operated as a compressor, the pressure differential may be established by feedback passages from the outlet port O to the end chambers, which may take the form of simple apertures in the side plates at the upper portions of the working chamber, although such a pressure differential is not always necessary. During operation as a pump the end chambers may simply be vented to atmosphere or sealed at atmospheric pressure, whereby a pressure differential is established by the negative pressure in the inlet port I. The rotor may be of the sliding radial vane type, and with such an arrangement a working fluid is pumped or compressed between the inlet port I and the outlet port O as the shaft 5 is rotationally driven.
Several problems have still been found to exist with this type of a rotary pump construction, however. More specifically, the rotor 6" is formed of cast iron and the side plates 7' and 8' are formed of a synthetic resin. According to the Japanese Utility Model application No. sho-50-26136, corresponding to British Pat. No. 1,515,635, the relationship between the inner diameter of the rotor chamber and the thickness of the side plates is described. In case the thickness of the side plates is less than the predetermined range, one part of the side plates which faces the compression stroke position is deformed toward the end chambers resulting in deteriorating sealability. While, in case the thickness of the side plates is larger than the predetermined range, the side plates may not sufficiently contact the end faces of the rotor in response to the pressure change of the rotor chamber if the pump is used as a vacuum pump also resulting in deteriorating sealability. In case the outer circumference of the side plates made of synthetic resin are embedded into the end heads, the side plates may excessively contact the end faces of the rotor due to thermal expansion of the side plates resulting in the side plates becoming excessively worn. Accordingly, the side plates may be frictionally stepped between the contacting and non-contacting locations. While in case the outer periphery of the side plates are not embedded into the end heads but just interposed between the stator housing and the end heads, a 0.6 mm stepped portion was created in the side plates when the experiment was made under the compression pump rotation of 6,000 rpm, after 500 hours of running. If the rotation is immediately reduced from 6,000 rpm to 500 to 1000 rpm, the exhaust flow is excessively reduced with time at 800 to 1,000 rpm, and therefore, such pump is not suitable to commercial use. It is apparent that if less than 0.2 mm stepped portion of the side plates is created after 500 hours of running at 6,000 rpm maintaining a high exhaust flow, such a pump would be practical for use in applications requiring rotation of 800 to 6,000 rpm. Thus, the side plates having a lower wear-resistance are subject to wear so as to be formed with stepped portions between its face in contact with the rotor and its face out of contact from the rotor after a great number of rotations. The stepped portions obstruct the radial movement of the vanes and thereby hinder the proper sliding movement of the vanes. This tendency frequently appears when the centrifugal force acting on the vane is small, that is, the rotor runs at a low rate in the range of about 500 to about 1,500 rpm. In the compression pump according to the present invention to be described hereinafter, the stepped portions of the side plates cannot be totally avoided, since the side plates are diaphragmatically contacted with the end faces of the rotor during the rotation of the rotor, and therefore, the effect caused by the stepped portion must be taken into consideration. Although one solution might be to make the side plates out of either ferrous or nonferrous metals, such side plates cannot be used without lubricant since, otherwise, seizure between the side plates and the rotor would result. Further even if both the side plates and the rotor are made of synthetic resin, the contacting pressure between the side plates and the rotor is increased due to their large thermal expansion, resulting in increasing the stepped portion and eventual thermal seizure. If the thin side plates are used to avoid this problem, then the above-mentioned drawbacks relating to deteriorating sealability due to deformation of the side plates will result.